In the periodical Applied Physics Letters, Volume 8 (1966), No. 12, pages 318-319, a pulsed metal ion laser is described by Silfvast, Fowles, and Hopkins, which operates with the metals cadmium, zinc, germanium, tin, lead, and indium as active laser materials in vapor form with a pulsed electric discharge.
Silfvast further described in Applied Physics Letters, Volume 15 (1969), No. 1, pages 23-25, a continuously operating ion laser with cataphoretic transport of the vapor of the elements cadmium, tin, and zinc. For excitation a continuous gas discharge in helium at a few mbar of pressure is used.
The continuously operating cataphoretic ion laser with the material selenium, that emits up to 24 laser lines in the visible region of the spectrum, made public by Silfvast and Klein in Applied Physics Letters, Volume 17 (1970), No. 9, pages 400-403, seemed particularly attractive. In this device a discharge vessel was used which made use of a discharge capillary of an inner diameter 4 mm made of pyrex glass and having a length of 1 m. For the strongest selenium ion laser lines at 522 nm an amplification of 5.4% per meter was measured.
After these first scientific publications it was sought to develop the cataphoretic metal ion laser further, to provide technically usable products. Heretofore, however, particularly for helium-selenium lasers, no industrially produced lasers were available that had adequate reliability of operation and a sufficient service life.
Above all two problems substantially impeded the development of the metal ion laser on the scale of industrial production:
1. The vapor stream produced by cataphoresis is not completely condensed at the cathode-side end of the discharge capillary and the vapor precipitates out on the exit window such as, for example, on a Brewster window. The internal losses are thereby greatly raised and the laser power thereby sinks steeply. The vapor stream can also reach the cathode where it greatly reduces the thermal emission (e.g. with selenium) and destroys the commonly used oxide cathodes.
2. The second problem concerns a great loss of the gas component of the gas-vapor mixture. Since for atomic reasons helium is preferably used as the gas for exciting the laser vapor, and helium unfortunately diffuses very easily through hot discharge capillaries, a strong helium pressure loss results and thereby also a rapid decrease of the laser power.
Heretofore, for raising the laser power, it was above all recommended to increase the active volume, here especially by prolongation of the discharge tube. Another theoretically conceivable way would be the raising of the discharge current density of the active medium: the required current densities for this purpose are not, however, adequately controllable and, moreover, the above described problems occur.